This invention relates to an improved method for the preparation of clay-bearing grease compositions.
In the past, as described by U.S. Pat. No. 3,006,848 for example, clay-bearing grease compositions are prepared by direct interchange of the clay from an aqueous phase to an oil phase by the addition of a hydrophobic amine which coats the surface of the clay converting the clay to one having an oleophilic hydrophobic surface. Further, as described in Canadian Pat. No. 731,131 (U.S. Pat. No. 3,222,279), polyepoxides are known to have been added to the clay dispersion to produce a resin-coated clay. As described in such Canadian Patent, the polyepoxide and amine are added to an acidified clay hydrogel. Curing is then accomplished to form a resin in the presence of an acid. This is followed by separating the water phase from the clay. The coated clay is then mixed with lubricating oil. As described in the Canadian Patent a small amount of strong mineral acid must be added to the clay dispersion prior to the incorporation of the polyepoxide resin. Water separation must take place after the resin is cured.
Canadian Pat. No. 913,053 describes avoiding the acidification step as described in Canadian Pat. No. 731,131 by utilizing a hydrophobic surfactant defined as "conjugate acids of aliphatic amines and aliphatic amine oxides."
Amines which form suitable conjugate acid surfactants include primary, secondary and tertiary amines having at least one long chain aliphatic group attached to the nitrogen atom, the total number of carbon atoms of said group or groups being from about 14 to about 24. Amines containing predominantly saturated, unbranched, aliphatic groups have been found to form extremely oxidation-resistant grease compositions. Amines of this type include tallow amines, especially hydrogenated primary, secondary, or tertiary tallow amines, e.g., dimethyl hydrogenated tallow (tertiary) amine and hydrogenated tallow (primary) amine. Saturated aliphatic primary amines having from 16 to 18 carbon atoms have been found to form particularly advantageous conjugate acid surfactants. Such amines are commercially available under the trademark Armeen HT.
Amine oxides suitable for forming the conjugate acid hydrophobing agents of the invention include primary, secondary and tertiary aliphatic amine oxides having a total of from about 14 to about 24 carbon atoms. Amine oxides which form especially advantageous conjugate acids are tertiary aliphatic amine oxides such as dimethyl alkyl amine oxides, particularly dimethyl hydrogenated tallow amine oxide.
The aforedescribed amines and amine oxides are converted to their conjugate acid form by reaction with an acid having a pKa value from 1 to 5. As used throughout this disclosure, pKa identifies the negative log of Ka which is the ionization constant defined by the equation: EQU Ka=([H+][A-])/[HA]
wherein HA is an acid, H+ is hydrogen ion and A- is an anion. Of the acids having pKa values within the above specified range, C.sub.1 to C.sub.4 aliphatic monocarboxylic acids form a preferred class of conjugate acids, particuarly acetic acid.
The aforesaid conjugate acid surfactants are preferably employed in the present compositions in relatively low concentrations, e.g., 30 to 60% by weight, more preferably, 30 to 40% by weight, based on the weight of the clay. Higher surfactant concentrations, for example, up to 125% by weight basis of the clay can be used, if desired. The use of such high concentrations of surfactant would, of course, be economically less attractive and for this reason are normally not employed.
The use of these materials are described to improve the clay-bearing grease over those previously used. The invention described herein is still yet another improvement in the method for preparing clay-bearing grease over those previously used.